A typical modern wireless communication transceiver is comprised of an IQ modulation transmitter and a quadrature receiver. The receiver includes a down-converter stage to translate the input Radio Frequency (RF) carrier signal to an Intermediate Frequency (IF) signal. The transmitter includes an up-converter stage to translate the IF input signal to RF output signal.
Specifically, a quadrature receiver implements complex down-conversion, whereby an RF signal is split and multiplied by a Local Oscillator (LO) and a quadrature version of the LO. In effect, in one branch of the receiver the input signal is multiplied by a cosine waveform at the LO frequency, while in the other branch the input signal is multiplied by a sine waveform at the LO frequency. The resulting outputs are known as in-phase (I) and quadrature (Q) IF components.
An IQ modulation transmitter implements complex up-conversion, whereby IQ IF input signals are multiplied by an LO and a quadrature version of the LO and then summed and amplified. In effect, in one branch of the transmitter the I-input signal is multiplied by a cosine waveform at the LO frequency, while in the other branch the Q-input signal is multiplied by a sine waveform at the LO frequency. The resulting output is an RF modulated signal.
Imperfections in the analog components of the up/down-converters and/or the LO paths and/or the transceiver IF paths can result in imbalances between the I and Q branches, with respect to their relative amplitudes, relative phases or both. Such imbalance is referred to as IQ Imbalance (IQI) which can have significant impact on the performance of a wireless transceiver. This is particularly true in direct-conversion receivers using OFDM modulation with high constellations contemplated for many future applications.
IQ Imbalance may occur when an amplitude and/or phase of I and Q components (of the IF branches or the LO branches) are mismatched. For example, when I and Q branches have phase difference which is not exactly 90° or when I and Q branches have non-equal amplitudes. Analog components that may contribute to the mismatch are mixers, IF amplifiers, IF filters, ADCs, DACs and others.
The IQ imbalance can also be frequency dependent. Such a mismatch is mainly contributed by the IF section.
Realistic values of mismatch (without calibration and with careful design) are 1°-5° in the phase difference and 0.1 dB-0.5 dB in the amplitude or gain difference. These numbers result in 20 dB-40 dB Image Rejection.
IQ imbalance has a direct impact on transmit (Tx) and receive (Rx) EVM and thus on Sensitivity. Usually we will refer to its contribution to the EVM as Image Rejection Ratio (IMR). EVM dependence on additional parameters, like linearity and Integrated Phase Noise emphasizes the impact of IQ imbalance on EVM. IQI is considered to be critical for OFDM modulation especially when a direct conversion receiver is implemented and higher data constellations are used.
Many algorithms for correcting IQ imbalance are implemented fully in Digital domain. Some do the IQ imbalance measurement in the digital domain but do the correction in the analog domain.